In recent years, there has been an increased interest in an automatic manual transmission for medium duty and heavy duty trucks, for example. An automatic manual transmission differs from the automatic transmission used in automobiles in that it has a positive clutch which when engaged has no slippage. The standard automatic transmission used in automobiles has some torque transfer loss and provides less than optimal vehicle mileage when compared to a clutch and a manual transmission operated by a skilled operator. In an automatic manual transmission, like the manual transmission, the clutch is disengaged to disconnect the motor from the transmission prior to shifting gears, the gears are shifted through movement of one or more levers, and the clutch is reengaged. Only with the automatic manual transmission the clutch and the gear shifting mechanism are operated with a hydraulic system and a gear shift controller.
The clutch and transmission shifting mechanism may be operated by pressurized hydraulic fluid. The hydraulic fluid pressure is obtained from a pump which is constantly driven by the vehicle motor. A pressure regulating valve is needed to maintain a desired fluid pressure over a range of pump speeds. Since the pump is constantly driven against the back pressure of the pumped fluid, there is a continuous energy loss in the system.
The clutch may be operated by a hydraulic cylinder and two solenoid actuated valves. To disengage the clutch, one valve is opened to deliver pressurized fluid to the clutch actuating cylinder The fluid pressure moves a piston which disengages the clutch to disconnect the vehicle motor from the transmission. When the clutch is to be reengaged, a second valve is opened to dump fluid from the clutch actuating cylinder. Various techniques have been suggested to provide a smooth reengagement which gradually brings the clutch output shaft up to the speed of the motor drive shaft Ideally, clutch reengagement is similar to the reengagement of a manual clutch by a skilled operator's foot.
According to one known technique, an expensive precision valve has been used for the second valve and the second valve is pulse width modulated to accurately control venting of fluid from the clutch actuating cylinder. A small volume of fluid is vented from the cylinder each time the valve is pulsed. The volume flow through the valve for each modulation step must be sufficiently low to require a number of modulation steps between the point that the clutch friction plates are just touching or "kissing" to when the clutch is fully engaged to produce a smooth clutch engagement. According to U.S. Pat. No. 4,535,879, the duty cycle of the pulse width modulation may be varied to provide the desired smooth clutch reengagement. According to another technique described in U.S. Pat. No. 4,533,030, a valve initially vents the clutch actuating cylinder to a control cylinder having an intermediate pressure to partially release the pressure in the cylinder and accordingly to partially reengage the clutch. Subsequently, the cylinder is fully vented by a throttle controlled valve and a bleed orifice to fully engage the clutch. U.S. Pat. No. 4,560,044 shows a valve system which provides an initial rapid fluid flow to a piston until a predetermined pressure is reached. The rate of fluid flow to the piston then is limited by diverting a portion of the flow to an accumulator to provide a more gradual fluid pressure increase until the clutch friction plates initially contact to slow the increase in pressure between the friction plates Subsequently, the fluid pressure is rapidly increased to complete the clutch engagement without slippage. A sophisticated valving arrangement is required to control the flow of fluid to the clutch piston.